Sm(cid:142) inaire LAPP , 11 octobre 2002 Polymer films characterized by Positron Annihilation Spectroscopy C. Bas, E. Pinel, S. Neyertz, R. Mercier, N.D. Alb(cid:142) rola Laboratoire Matr(cid:142) iaux Organiques (cid:136) Proprit(cid:142) s(cid:142) Sp (cid:142) cifiques (LMOPS) - UMR 5041 M.-F. Barthe, P. Desgardin Centre d’Etudes et de Recherches par Irradiation (CERI) - UPR33 1 Background In a polymer, different types of positrons : Positron-Electron pair (e+,e-) Free Positron (e+) Positronium Ps pPs oPs Annihilation Intrinsic Annihilation Pick-off Annihilation g g g g g g e++ e- (cid:25) 2 g Lifetime t = 0.125 ns 1 Lifetime t = 0.4 ns 2 Lifetime t > 0.5 ns 3 Intrinsic annihilation g Lifetime t = " In vacuum g 2 g Lifetime t = 142 ns 3 2 t and t are constant 1 2 t depends on the polymer microstructure 3 Electron cloud - 1 Ø R 1 (cid:230) 2¶ (cid:215) R (cid:246) ø (cid:153) = 0.5(cid:215) Œ 1- + sin(cid:231) (cid:247) œ 3 º R + D R 2¶ Ł R + D R ł ß oPs 2.2 (cid:129) R D R D R : electron layer thickness around the hole = 1.66 (cid:129) Bulk polymer Tao-Eldrup model [*] => detection of holes with diameter ranged from 3 (cid:129) to 15 (cid:129) [*] J. Chem. Phys.(1972) 56, 5499 3 Chem. Phys. (1981) 63, 51 Birth g -ray Background qqqq 1.27 MeV DDDD t = tttt g -ray annihilation E = 0.511 MeV – DDDD E Sample Na22 (cid:25) Ne22 + e+ + g Positron source 22Na Diffusion (100nm) g -ray annihilation E = 0.511 MeV – DDDD E Thermalization (1ps) (cid:255) Measurement of positron lifetime DDDD t = tttt (Positron Annihilation Lifetime Spectroscopy - PALS) (cid:255) Measurement of energy broadening DDDD E = p c/2 z (Doppler Broadenning Spectrocopy - DBS) (cid:255) qqqq Measurement of angle (Angular Correlation of Annihilation Radiation - ACAR) 4 Apparatus (cid:255) Measurement of positron lifetime DDDD t = tttt (Positron Annihilation Lifetime Spectroscopy - PALS) Example of spectrum PMT : Photomultiplicateur CFDD : Constant fraction differential discriminator From this spectrum, it is possible to determine : TAC : Time to amplitude converter I , t (cid:247) pPs ADC : Analogue-to-digital converter 1 1 I , t (cid:247) e+ 2 2 MAC : mulitchannel analyzer I , t (cid:247) oPs 3 3 5 Background Origin of energy broadening DDDD E : (cid:255) is related to the bound energy of the annihilation electron (or momentum e +,e- pair) ¨ if the free positron annihilates with a valence electron, D E is low ¨ if the free positron annihilates with a core electron, D E is high => broadening effect Positron-Electron pair (e+,e-) Free Positron (e+) Positronium Ps pPs oPs Annihilation Instrinsic Annihilation Pick-off Annihilation g g g g g g e++ e- (cid:25) 2 g Narrow Doppler line Broad Doppler line Broad Doppler line 6 DBS peak S parameter: S = As / At ˛ S mainly related to annihilations with valence electrons ˛ structural informations (defects) s As t n u o W parameter: C W = Aw / At Aw Aw ˛ W mainly related to annihilations with core electrons ˛ Chemical informations 504 506 508 510 512 514 516 518 Energy (keV) S-parameter W-parameter 0,59 0,20 0,58 0,19 0,57 0,18 0,56 0,17 32 34 36 38 40 42 44 7 oPs intensity - I (%) 3 Slow positron beam Positron source of 22Na : maximum of energy of 540Kev The mean implantation depth of the positrons ( z ) depends on the incident positron energy (E) o 400 z(A) = . E(keV)1.6 d For polymer with d =1.3 : 540 keV · 700 m m 25 keV · 5 m m Cao et al. Appl. Surf. Sci. (1999), 149, 116 8 French positron facility for materials science Strasbourg Ires --- Arcueil PALS DGA - ? --- --- milieux micellaires 1 chercheur ---- 1 chercheur Chambr(cid:142) y / Annecy LMOPS / LAPP --- Orla(cid:142) ns PALS (fin 2002) CERI --- --- No slow positron pulsing system matr(cid:142) iaux peu Positron beam coupled with denses DBS + PALS with 22Na source --- Semi-conducteurs ---- 2 chercheurs 9 Our results CERI facility : positron beam with positron energy from 0.5 keV to 25 keV coupled with DBS Preliminary results with PALS from Perkin Elmer (to be confirmed) Samples : films of 20 to 30 m m thickness polyimides O ODA-CF3 O ODOPA O O BCDA O C(CH ) O C(CH ) 33 33 N O N N O N N O N x y n O CF O O CF O O CF O 3 3 3 Mean depth (m m) 0 0.4 1.2 2.3 3.6 5.2 0.47 0.050 BCDA-ODA O O 0.048 N N O r e r t 0.46 e O O e t m 0.046 e m a r a a r p 0.044 a - p S 0.45 - W 0.042 0.44 0.040 0 5 10 15 20 25 10 Energy (keV)